The question has been studied numerous times in the past, yielding sometimes conflicting results. But the new study -- systematic review and meta-analysis of published data -- is the largest to date.

"As currently being used, e-cigarettes are associated with significantly less quitting among smokers," wrote first author Dr. Sara Kalkhoran, who was a clinical fellow at the UCSF School of Medicine when the research was conducted. "E-cigarettes should not be recommended as effective smoking cessation aids until there is evidence that, as promoted and used, they assist smoking cessation." Kalkhoran is now at Massachusetts General Hospital and Harvard Medical School.

For the current study, Kalkhoran and her team reviewed 38 studies assessing the association between e-cigarette use and cigarette cessation among adult smokers. They selected 20 studies that had control groups of smokers not using e-cigarettes, and combined their results in a meta-analysis.

They found that the odds of quitting smoking were 28 percent lower among smokers who used electronic cigarettes when compared to those who did not.

"The irony is that quitting smoking is one of the main reasons both adults and kids use e-cigarettes, but the overall effect is less, not more, quitting," co-author Stanton A. Glantz, PhD, UCSF professor of medicine and director of the UCSF Center for Tobacco Control Research and Education, told CBS News. "While there is no question that a puff on an e-cigarette is less dangerous than a puff on a conventional cigarette, the most dangerous thing about e-cigarettes is that they keep people smoking conventional cigarettes."